High temperature alloy

ABSTRACT

A nickel-base superalloy possessed of excellent elevatedtemperature strength properties in an as-cast condition, as well as excellent resistance to oxidation and sulfidation attack upon exposure to high temperature corrosive environments such as encounted in the hot section of gas turbine engines. The superalloy is comprised of from about 5 to about 7% chromium, about 12 to about 20% molybdenum, about 0.5 to about 1.5% hafnium, about 6.5 to about 7.5% aluminum, about 0.01 to about 0.20% carbon, up to about 15% cobalt, and the balance nickel and conventional residuals and impurities.

United States Patent [191 Morrow, III et al.

[ Dec. 17, 1974 HIGH TEMPERATURE ALLOY [75] Inventors: Hugh Morrow, III; David L.

Sponseller, both of Ann Arbo Mich.

[73] Assignee: Americal Metal Climax, Inc., New

York, NY.

[22] Filed: Mar. 15, 1974 [21] Appl. No.: 451,439

[52] U.S. Cl 75/171, 148/32, 148/325 [51] Int. Cl. C22c 19/00 [58] Field of Search 75/171, 170; 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 3,677,747 7/1972 Lund ct al. 75/171 Primary Examiner-R. Dean Attorney, Agent, or Firml-larness, Dickey & Pierce [5 7] ABSTRACT A nickel-base superalloy possessed of excellent elevated-temperature strength properties in an as-cast condition, as well as excellent resistance to oxidation and sulfidation attack upon exposure to high temperature corrosive environments such as encounted in the hot section of gas turbine engines. The superalloy is comprised of from about 5 to about 7% chromium, about 12 to about 20% molybdenum, about 0.5 to about 1.5% hafnium, about 6.5 to about 7.5% aluminum, about 0.01 to about 0.20% carbon, up to about 15% cobalt, and the balance nickel and conventional residuals and impurities.

6 Claims, N0 Drawings BACKGROUND OF THE INVENTION The present invention is directed to so-called nickelbase superalloys of the type which are of high strength at elevated temperatures, rendering them particularly suitable for use in the fabrication of parts or components for high performance gas turbine engines or the like. A variety of superalloys suitable for use in an ascast condition have heretofore been used or proposed for use for fabricating gas turbine engine components employing investment or like casting techniques, but have been unsatisfactory for one or a number of reasons. While many of such alloys are of adequate elevated temperature strength, a continuing problem has been their susceptibility to oxidation and sulfidation attack when employed as components in aircraft gas turbine engines due to the presence of the sulfur constituents in the fuel employed which upon combustion in the presence of sodium chloride from sea air produce a molten salt mixture which attacks the metal alloy causing an accelerated deterioration thereof. It is also a desirable attribute of such alloys to enable a controlled orientation of their grain structure by directional solidification techniques, thereby achieving optimum elevated temperature strength properties of the cast component.

The problems and disadvantages associated with prior art castable nickel-base superalloys are overcome in accordance with the alloy of the present invention incorporating carefully controlled amounts of chromium, molybdenum, hafnium and aluminum in a base consisting predominantly of nickel, thereby achieving not only optimum elevated temperature strength properties, but also excellent resistance to oxidation and sulfidation attack when employed in the hot section of gas turbine engines, such as for vanes and turbine blades.

SUMMARY OF THE INVENTION The benefits and advantages of the present invention are achieved by an improved nickel-base superalloy composition containing about to about 7% chromium, about 12 to about molybdenum, about 0.5 to about 1.5% hafnium, about 6.5 to about 7.5% aluminum, about 0.01 to about 0.20% carbon, up to about 15% cobalt, and the balance consisting of nickel along with residuals and incidental impurities present in conventional amounts. A particularly satisfactory alloy nominally comprises about 6% chromium, about 14% molybdenum, about 7% aluminum, about 1% hafnium, about 0.1 1% carbon, and the balance substantially all nickel.

Alloys of the foregoing compositions are castable to relatively close final dimensional tolerances such as by investment casting techniques, and are capable of being directionally solidified imparting a grain orientation in a direction conforming to the direction of stress or loading of the part in actual use, thereby optimizing the physical strength performance of the component. The superalloy is further characterized by a relatively low coefficient of thermal expansion, assuring maintenance of close dimensional tolerances in spite of wide temperature fluctuations associated with gas turbine engine operation, and further possesses excellent resistance to oxidation and sulfidation attack in the corrosive environments typified by gas turbine engine combustion gases.

Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments, taken in conjunction with the specific examples provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The amounts and proportions of the several alloying constituents comprising the superalloy of the present invention are described in terms of percentages by weight in the specification and the subjoined claims, unless expressly indicated otherwise.

The essential alloying constituents contained in the superalloy composition of the present invention, as well as the broad useable proportions and preferred amounts, are set forth in Table 1:

The quantity of carbon is controlled with a range of 0.01 up to 0.20%, and preferably from 0.1 to 0.15%, since quantities greater than about 0.2% result in excessiveembrittlement of the alloy, while quantities less than about 0.01% do not provide adequate strength.

The chromium constituent adds strength as well as oxidation and sulfidation resistance to the alloy, and in combination with the aluminum constituent, controls the oxidation mechanism as well as the type of surface oxides formed. Amounts of chromium less than about 5% result in an alloy which does not possess the requisite oxidation and sulfidation resistance. On the other hand, quantities of chromium employed in amounts in excess of about 7% result in phase instability of the alloy and causes a formation of the undesirable a phase comprising an intermetalic compound consisting of nickel, molybdenum and chromium conventionally designated as A 8 Quantities of chromium in excess of the maximum amount designated in Table 1 also result in a loss of strength in the alloy and an undesirable lowering of the gamma prime solvus temperature.

The molybdenum constituent employed in the amounts specified in Table 1 contributes to the excellent high temperature strength characteristics of the superalloy, and to the alloys relatively low coefficient of thermal expansion, comprises a gamma prime phase former and also enhances the oxidation and sulfidation resistance of the alloy in combination with the chromium constituent. Amounts of molybdenum generally less than about 12% result in inadequate strength of the alloy and a higher than desirable coefficient of thermal expansion. On the other hand, amounts of molybdenum in excess of about 20% result in phase instability of the alloy, as well as undesirable brittleness and a loss of its ductility.

Hafnium utilized in the small controlled amounts set forth in Table l imparts gamma prime forming characteristics and carbide forming characteristics to the alloy and dissolves in the gamma prime phase, thereby adding strength to the alloy and also increasing its ductility and corrosion resistance. The utilization of hafnium in amounts above about 1.5% has been found to reduce the oxidation resistance of the alloy and to result in undesirable microstructural features. On the other hand, quantities of hafnium less than about 0.5% have been found to reduce the oxidation resistance, stress rupture strength and ductility of the alloy below acceptable levels.

The aluminum constituent is a gamma prime former and provides the adherent oxide which contributes to the oxidation resistance of the alloy. Quantities of aluminum less than about 6.5% result in a reduction in the volume proportion of gamma prime phase formed accompanied by a reduction in the strength and in the corrosion resistance of the alloy. Quantities of aluminum in excess of about 7.5% results in a progressive formation of a beta phase comprised of an intermetallic compound consisting of nickel and aluminum which is a weaker phase than the gamma prime phase, thereby resulting in a reduction in the strength of the alloy. The specific amount of aluminum employed is controlled in consideration of the quantity of chromium incorporated in the alloy, providing a balanced system which imparts optimum oxidation and sulfidation resistance to the alloy consistent with the intended end use thereof.

In addition to the foregoing specific alloying constituents, the superalloy may further contain cobalt as a substitute for a portion of the nickel in an amount generally not greater than about of the alloy. Amounts above this magnitude adversely affects the stability of the gamma prime, which in turn results in a corresponding reduction in the strength of the alloy. The use of cobalt as a substitute for part of the nickel is generally undesirable from an economical standpoint due to the higher cost of cobalt. in accordance with the preferred practice, cobalt is employed in the residual amounts normally present in commercially available nickel.

The balance of the alloy composition consists essentially of nickel together with incidental impurities and residuals present in conventional amounts. Such normal residuals and incidental impurities can be tolerated in quantities which do not adversely affect the elevated temperature strength properties and the resistance of the alloy to oxidation and sulfidation attack as exemplified by the hot combustion gases of gas turbine engines. in accordance with the preferred practice. sulfur, phosphorus and other known deleterious incidental impurities should be maintained as low as possible.

The superalloy composition can be prepared employing vacuum melting techniques and the resultant heat can readily be cast into molds employing an inert atmosphere such as an argon gas atmosphere. The alloy of the present invention is particularly responsive to directional solidification, whereby a directional orientation in the microstructure of the resultant solidified casting can be achieved so as to correspond with the direction or axis of maximum load or stress, thereby optimizing the physical strength properties of the ascast component. By carefully controlled directional solidification techniques, depending upon the specific configuration of the casting, it is feasible to produce monocrystal components which are of optimum strength.

EXAMPLE An experimental heat of the superalloy was prepared having a composition as follows: carbon 0.12%; chromium 6.51%; molybdenum 14.0%; hafnium 1.05%; aluminum 7.03%; and the balance nickel. The charge materials employed were electrolytic nickel (99.85% Ni), electrolytic chromium (99.57% Cr), pressed-andsintered molybdenum pellets (99.70% Mo), aluminum shot (99.85% Al), hafnium sponge (96.36% Hf3.30% Zr), and spectrographic grade graphite (essentially C). The charge materials were vacuum-induction melted employing a procedure in which the nickel, chromium and molybdenum charge materials are first melted at a vacuum of about 15 microns, whereafter the heat is superheated 200F above the melting temperature and argon is introduced into the melting chamber at a pressure of 250 mm. Thereafter, the aluminum is added with an allowance of an extra 0.05% for deoxidizing the melt and whereafter the required carbon and hafnium are added.

Approximately five minutes were allowed after the last addition to assure that all constituents had fully dissolved and homogenized, whereafter experimental ingots were poured under an argon atmosphere in the melt chamber and were cooled for about 10 minutes before exposure to the ambient atmosphere. The test ingots were approximately 3% inch (89 mm) in diameter by 8 inches (203 mm) long. The cast ingots were cropped to remove a coupon for chemical analysis and hot top, and thereafter were remelted and vacuum cast into test bar clusters, each consisting of 16 cast-to-size threaded stress-rupture and tensile specimens and two quantitative analysis bars that were employed for chemical analysis, thermal expansion specimens and microstructural studies.

The tensile test specimens measured 3% inch (89 mm) long by k inch (13 mm) diameter, and were threaded, with a 1% inch (32 mm) gauge length and a 0.25 inch (6.35 mm) gauge diameter. Room temperature, 1400F and 2000F tensile properties of the tensile test specimens were evaluated in the as-cast condition at .a strain rate of 0.005 per minute through the 0.2% yield stress and 0.05 per minute thereafter to failure. The tensile testing was carried out in accordance with the provisions of ASTM Standards E 8-69, Tension Testing of Metallic Materials, and E 21-70, Elevated Temperature Tension Tests of Metallic Materials. The results of the tensile strength tests are set forth in Table 2.

TABLE 2 Tensile Strength Properties The stress-rupture properties of the tensile test specimens were also determined at 1600F, 1800F and 2000F in the ascast condition in accordance with the provisions of ASTM Standard E l3970, Conducting Creep, Creep-Rupture and Stress-Rupture Tests of Metallic Materials. These data are set forth in Table 3.

TABLE 3 Stress-Rupture Properties els of magnification. The microstructure of the alloy can generally be characterized as consisting of primary dendrites of the gamma-solid solution having a high volume fraction of the gamma prime precipitate and a very fine phase of so-called Chinese script" alphamolybdenum which is not completely surrounded by 200 Hr. Rupture Elong- Reduction Rupture Temperature Stress Life. ation, in Area. Stress F C ksi N/mm hrs. ksi N/mm 15 103 110.8 1.5 1.5 12.9 89 137 31.2 2.5 2.0 2000 1095 4.5 31 147.3 nil nil 6 41 66.9 nil nil 4.2 29 8 55 27.0 nil nil 8 55 23.8 1.0 1.0

As will be noted, duplicate stress-rupture tests were conducted at 2000F at a stress of 8 ksi for the purpose of checking data reproducibility.

The mean linear thermal expansion coefficients from room temperature to a series of elevated temperatures were determined with a Leitz Universal Dilatometer Model UBD. The test specimens in the as-cast condition were heated from room temperature at a rate of 45F (C) per minute in an argon atmosphere to each of a series of designated elevated temperatures and themean linear thermal expansion coefficients were calculated directly from the change of length as a function of temperature. The data as reported in Table 4 comprises the average of four expansion coefficient determinations starting at room temperature to the designated temperature on the individual specimens.

TABLE 4 Themial Expansion Coefficients Mean Linear Thennal Expansion Temperature, "C Coefficient, 10 per C An examination of the microstructure of the alloy in the as-cast condition was also conducted at various levthe gamma prime envelope but is observed to be surrounded by or adjacent to a gamma-gamma prime eutectic in the interdendritic regions and primary gamma prime patches. No intermetallic compound of p. phase was observed in the microstructure of the alloy in the as-cast condition.

An examination of the microstructure of the alloy was also made after a 1,000 hour exposure at 1500F (815C) to observe any microstructural changes or instability in the phases. lt wasobserved that no general precipitation of the p. phase occured confirming the stability of the alloy with respect to the formation 'of new phases and with the only significant microstructural change comprising a normal coarsening of the gamme prime phase.

These tests and the data as herein described and as set forth in Tables 2-4 clearly substantiate the excellent elevated temperature tensile and stress rupture strengths of the alloy of the present invention in the ascast condition, the relatively small coefficient of thermal expansion rendering the alloy eminently suitable for use in fabricating precision parts or components for precision gas turbine engines. The alloys are of a substantially stable microstructure even when subjected to elevated temperatures for prolonged time periods and possess excellent resistance to corrosion and sulfidation attack.

While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages as set forth above, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

about to about 7% chromium, about 12 to about 20% molybdenum, about 0.5 to about 1.5% hafnium, about 6.5 to about 7.5% aluminum, about 0.01 to about 0.20% carbon, up to about 15% cobalt and the balance consisting essentially of nickel together with normal residuals and incidental impurities present in conventional amounts.

2. The nickel-base alloy as defined in claim 1, in which chromium is present in an amount of about 5.5 to about 6.5%, molybdenum is present in an. amount of about 13.5 to about 14.5%, halfnium is present in an amount of about 0.7 to about 1.25%, aluminum is present in an amount of about 6.75 to about 7.25%, and carbon is present in an amount of about 0.1 to about 0.15%. 1

3. The alloy as defined in claim 1 in which cobalt is present as a residual in an amount normally encountered in commercial nickel.

4. The alloy as defined in claim 1, nominally containing about 6% chromium, about 14% molybdenum, about 1% hafnium, about 7% aluminum, about 0.1 1% carbon and with the balance consisting essentially of nickel and incidental impurities and normal residuals present in conventional amounts.

5. The alloy as defined in claim 1, in the as-cast condition.

6. The alloy as defined in claim 1, in the as-cast directionally solidified condition characterized in having a grain structure oriented in the direction of solidification of the alloy.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3 54 94 DATED December 17, 1974 INVENTOHS) Hugh Morrow, III et al it is certified that error appears m the above-identified patent and that sard Letters Patent are hereiry corrected as shown b low;

Abstract, line 5, 'encounced" should be --encountered-. Column 1, line 19, "fuel" should be --fuels--. Column 5, line 14, "ascast" should be --as-cast--. Column 7, line 14, I'0. 7" should be -0. 75--.

Signed and Scaled this twenty-second Day Of July 1975 [SEAL] A ttest:

RUTH C. MASON Altzsling Offl'rer C. MARSHALL DANN Commixxioner of Parents and Trademarks 

1. A NICKEL-BASE SUPERALLOY CONSISTING ESSENTIALLY OF ABOUT 5 TO ABOUT 7% CHROMIUM, ABOUT 12 TO ABOUT 20% MOLBDENUM, ABOUT 0.5 TO ABOUT 1.5% HAFNIUM, ABOUT 6.5 TO ABOUT 7.5% ALUMINUM, ABOUT 0.01 TO ABOUT 0.20% CARBON, UP TO ABOUT 15% COBALT AND THE BALANCE CONSISTING ESSENTIALLY OF NICKEL TOGETHER WITH NORMAL RESIDUALS AND INCIDENTAL IMPURITIES PRESENT IN CONVENTIONAL AMOUNTS.
 2. The nickel-base alloy as defined in claim 1, in which chromium is present in an amount of about 5.5 to about 6.5%, molybdenum is present in an amount of about 13.5 to about 14.5%, halfnium is present in an amount of about 0.7 to about 1.25%, aluminum is present in an amount of about 6.75 to about 7.25%, and carbon is present in an amount of about 0.1 to about 0.15%.
 3. The alloy as defined in claim 1 in which cobalt is present as a residual in an amount normally encountered in commercial nickel.
 4. The alloy as defined in claim 1, nominally containing about 6% chromium, about 14% molybdenum, about 1% hafnium, about 7% aluminum, about 0.11% carbon and with the balance consisting essentially of nickel and incidental impurities and normal residuals present in conventional amounts.
 5. The alloy as defined in claim 1, in the as-cast condition.
 6. The alloy as defined in claim 1, in the as-cast directionally solidified condition characterized in having a grain structure oriented in the direction of solidification of the alloy. 